This document provides an overview of meiosis and sexual reproduction. It begins with a comparison of mitosis and asexual reproduction, which produce identical daughter cells, versus the need for sexual reproduction to generate genetic variation. Meiosis is introduced as a two-step cell division process that reduces the chromosome number from diploid to haploid to produce gametes like eggs and sperm. Homologous chromosome pairing and crossing over in meiosis are described as sources of genetic variation. The lifecycles of sexual reproduction, involving meiosis, fertilization, and mitosis are depicted. Key stages and events of meiosis one and two are outlined. Differences between mitosis and meiosis and their purposes are summarized. Sexual reproduction is concluded

1. Meiosis &
Sexual Reproduction
AP Biology
2007-2008

2. Cell division / Asexual reproduction
 Mitosis

produce cells with same information
 identical daughter cells

exact copies
 clones

same amount of DNA
 same number of chromosomes
 same genetic information
AP Biology
Aaaargh!
I’m seeing
double!

3. Asexual reproduction
 Single-celled eukaryotes
yeast (fungi)
 Protists

 Paramecium
 Amoeba
 Simple multicellular
eukaryotes

Hydra
What are the
disadvantages of
asexual reproduction?
AP Biology
What are the
advantages?
budding
budding

4. How about the rest of us?
 What if a complex multicellular organism
(like us) wants to reproduce?

joining of egg + sperm
 Do we make egg & sperm by mitosis? No!
What if we did, then….
46
egg
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+
46
92
sperm
zygote
Doesn’t work!

5. Human female karyotype
46 chromosomes
23 pairs
AP Biology

6. Human male karyotype
46 chromosomes
23 pairs
AP Biology

7. Homologous chromosomes
 Paired chromosomes

both chromosomes of a pair carry “matching” genes
 control same inherited characters
 homologous = same information
diploid
2n
2n = 4
AP Biology
single stranded
homologous
chromosomes
double stranded
homologous chromosomes

8. How do we make sperm & eggs?
 Must reduce 46 chromosomes → 23

must reduce the number of chromosomes by half
23
46
meiosis
46
AP Biology
zygote
23
egg
46
23
23
sperm
gametes
fertilization

9. Meiosis: production of gametes
 Alternating stages

chromosome number
must be reduced
haploid
 diploid → haploid
 2n → n
 humans: 46 → 23
 meiosis reduces
chromosome number
 makes gametes

fertilization restores
chromosome number
 haploid → diploid
 n → 2n
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diploid

10. Sexual reproduction lifecycle
 2 copies
 diploid
 2n
 1 copy
 haploid
 1n
gametes
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fertilization
meiosis
In the next
generation…
We’re mixing
things up here!
A good thing?
 1 copy
 haploid
 1n
gametes

11. Meiosis
 Reduction Division



special cell division for
sexual reproduction
reduce 2n → 1n
diploid → haploid
 “two” → “half”

makes gametes
 sperm, eggs
Warning: meiosis evolved from mitosis, so stages
& “machinery” are similar but the processes are
AP radically different. Do not confuse the two!
Biology

12. Overview of meiosis
I.P.M.A.T.P.M.A.T
2n = 4
interphase 1
prophase 1
metaphase 1
anaphase 1
n=2
n=2
prophase 2 metaphase 2 anaphase 2 telophase 2
n=2
telophase
AP Biology
1

13. Double division
of meiosis
DNA replication
Repeat
I can’t
after you!
hear me
AP Biology
1st division of
meiosis separates
homologous pairs
2nd division of
meiosis separates
sister chromatids
Meiosis 1
Meiosis 2

14. Preparing for meiosis
 1st step of meiosis
Duplication of DNA
 Why bother?

 meiosis evolved after mitosis
 convenient to use
2n = 6
single
stranded
“machinery” of mitosis
 DNA replicated in
S phase of interphase
of MEIOSIS
(just like in mitosis)
AP Biology
M1 prophase
2n = 6
double
stranded

15. Meiosis 1
 1st division of meiosis
2n = 4
single
stranded
separates homologous pairs
prophase 1
2n = 4
double
stranded
metaphase 1
2n = 4
double
stranded
synapsis
tetrad
reduction
telophase 1
I can’t
Repeat
AP Biology after you!
hear me
1n = 2
double
stranded

16. Meiosis 2
 2nd division of meiosis
1n = 2
double
stranded
separates sister
chromatids
prophase 2
What does
this division
look like?
1n = 2
single
stranded
AP Biology
1n = 2
double
stranded
metaphase 2
4
telophase 2

17. Steps of meiosis
 Meiosis 1
interphase
 prophase 1
 metaphase 1
 anaphase 1
 telophase 1

 Meiosis 2
prophase 2
 metaphase 2
 anaphase 2
 telophase 2
AP Biology

1st division of
meiosis separates
homologous pairs
(2n → 1n)
“reduction division”
2nd division of
meiosis separates
sister chromatids
(1n → 1n)
* just like mitosis *

18. Meiosis 1 & 2
AP Biology

19. Trading pieces of DNA
 Crossing over

during Prophase 1, sister
chromatids intertwine
 homologous pairs swap
pieces of chromosome
 DNA breaks & re-attaches
synapsis
tetrad
AP Biology
prophase 1

20. Crossing over
 3 steps
What are the
advantages of
crossing over in
sexual reproduction?
cross over
 breakage of DNA
 re-fusing of DNA

 New combinations of traits
AP Biology

21. Mitosis vs. Meiosis
AP Biology

22. Mitosis vs. Meiosis
 Mitosis






AP Biology
1 division
daughter cells
genetically identical
to parent cell
produces 2 cells
2n → 2n
produces cells for
growth & repair
no crossing over
 Meiosis






2 divisions
daughter cells
genetically different
from parent
produces 4 cells
2n → 1n
produces gametes
crossing over

23. Putting it all together…
meiosis → fertilization → mitosis + development
gametes
46
meiosis
46
23
egg
23
23
23
zygote
fertilization
AP Biology
sperm
46
46 46
46 46 46
46 46
46
mitosis
development

24. The value of sexual reproduction
 Sexual reproduction introduces genetic variation

genetic recombination
 independent assortment of chromosomes
 random alignment of homologous chromosomes in Metaphase 1

crossing over
 mixing of alleles across homologous chromosomes

random fertilization
 which sperm fertilizes which egg?
 Driving evolution

providing variation for natural selection
metaphase1
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25. Variation from genetic recombination
 Independent assortment of chromosomes
meiosis introduces genetic variation
 gametes of offspring do not have same
combination of genes as gametes from
parents

 random assortment in humans produces
223 (8,388,608) different combinations in gametes
AP Biology
from Mom
from Dad
offspring
new gametes
made by offspring

26. Variation from crossing over
 Crossing over creates completely new
combinations of traits on each chromosome

creates an infinite
variety in gametes
AP Biology

27. Variation from random fertilization
 Sperm + Egg = ?

AP Biology
any 2 parents will produce a zygote with
over 70 trillion (223 x 223) possible diploid
combinations

28. Sexual reproduction creates variability
Sexual reproduction allows us to maintain both
genetic similarity & differences.
Jonas
Brothers
AP Biology
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez

29. Sperm production
Epididymis
Testis
Coiled
seminiferous
tubules
germ cell
(diploid)
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
spermatozoa
 Spermatogenesis
continuous & prolific process
Cross-section of
seminiferous tubule
 each ejaculation =
100-600 million sperm
AP Biology

MEIOSIS II

30. Egg production
 Oogenesis




eggs in ovaries halted
before Anaphase 1
Meiosis 1 completed
during maturation
Meiosis 2 completed
unequal divisions
after fertilization
1 egg + 2 polar bodies
Meiosis 1 completed
during egg maturation
What is the advantage of
this development system?
Meiosis 2 completed
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triggered by fertilization
ovulation

31. Putting all
your egg
in one basket!
Oogenesis
germinal cell
(diploid)
primary follicles
fallopian tube
fertilization
primary
oocyte
(diploid)
MEIOSIS I
secondary
oocyte
(haploid)
first polar body
MEIOSIS II
after fertilization
second
polar body
AP Biology
ovum
(haploid)
developing
follicle
mature follicle with
secondary oocyte
ruptured follicle
(ovulation)
corpus luteum

32. Differences across kingdoms
 Not all organisms use haploid & diploid
stages in same way


which one is dominant (2n or n) differs
but still alternate between haploid & diploid
 must for sexual reproduction
AP Biology

33. Any Questions??
What are the
DISadvantages of
sexual reproduction?
AP Biology
2007-2008

34. Ghosts of Lectures Past
(storage)
AP Biology
2007-2008

35. AP Biology

36. AP Biology